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Re: [Xen-users] domU kernel

To: Christoph Purrucker <xen@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] domU kernel
From: Nico Kadel-Garcia <nkadel@xxxxxxxxx>
Date: Mon, 15 Oct 2007 09:42:31 +0100
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
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Christoph Purrucker wrote:
Hi Nico,

There's a big advantage to pygrub: kernel update procedures for DomU have
nothing to do with Dom0's kernel. This prevents version and feature
conflicts with package management sytems, and allows updates and reboots
of DomU without having to write to Dom0.

I like it the other way round: I do one kernel-update in Dom0 and all my
virtual machines profit by a simple DomU reboot. My VMs have no kernel and
no modules, so there is one source of error less for DomU-admins. No
kernel manipulations, no root-kits.
It doesn't work with RHEL 4 guest domains on a RHEL 5 server. You can't install the kernel-xenU on the RHEL 5 server without putting in override commands, and *that* causes compatibility whinging when you try to update kernel-xen. And then you have to go hand-edit the DomU config files, unless you're using the symlink based names for the kernel, and *that* is sensitive to which kernel you installed last.

Not having modules in the DomU's means that you can't do FUSE based file-system mounting in your DomU's, mount NTFS partitions, or do other useful tricks, unless you go to the extra pain of hand-building and hard-loading *all* the necessary modules on your Dom0 in order to have entirely consistent DomU's. Man, that's a lot of work! And you can't run DomU's with different kernels without extra management steps.

This was particularly troublesome with the Xensource kernels for RHEL 4
and RHEL 5, which had such different versions for kernel-xen on the
server and kernel-xenU on the guest that it made RHEL whine quite hard
about the Dom0 getting such an old kernel installed on it. And if you
tried to use kernel-xen on the DomU, it created conflicts because the
RHEL 4 kernel and RHEL 5 kernels had the same names and file names, and
it could screw up which kernel you wound up with.

This is a Redhat problem, evil, and not a reason to use pygrub.

No, it's an RPM problem. It's inherent in management systems that correctly detect version incompatibilities, and don't expect you to install software components on your Dom0 that are only there for DomU's, not for local use. It's even worse if you have a 64-bit Dom0 and 32-bit DomU's!!!

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